This invention relates generally to an optical signal source for a fiber optic interferometric sensor. More particularly, this invention relates to an optical signal source for a fiber optic rotation sensor. Still more particularly, this invention relates to an optical signal source that includes a gain fiber that is optically pumped to produce a broadband optical signal. The invention provides apparatus for eliminating backward reflections of the pump light in a double-pass optical pumped gain fiber.
A double-pass optical pumped gain fiber produces greater signal intensity than can be obtained with a single pass of the pump light through a gain fiber. The gain fiber normally is doped with erbium, that produces gain in the optical signal frequency used in fiber optic rotation sensors.
However, use of a double-pass broadband fiber source in a fiber optic gyroscope has been limited. Problems encountered with double-pass optical pumping include gain depletion within the erbium fiber due to feedback from the gyro, crosstalk between gyro axes and the onset of lasing when pumping the erbium doped fiber with an intense pump laser diode in order to get a high power broadband fiber source. This lasing problem becomes more significant when the erbium fiber is made of a short length. Short length double-pass systems are desirable when trying to minimize effects due to exposure of the erbium fiber to ionizing radiation.
The typical broadband fiber source used in fiber optic gyros is a reverse pumped, single-pass fiber source. This configuration uses a laser diode that emits light at a given wavelength. This light is directed through a wavelength division multiplexer, WDM, that has two input leads and two output leads. One of the output leads of the WDM is physically connected to a length of erbium doped fiber, EDF. The EDF is terminated at one end with an angle capillary tube that keeps the light from being reflected back into the fiber. The EDF has a core that has been doped such that spontaneous emission occurs when light of a specific wavelength and sufficiently high intensity is launched into the core. This emission occurs in both directions of the EDF. In the reverse pumped single-pass configuration, light from the WDM is directed into the EDF. The EDF then emits light in both directions. The forward directed light exits the EDF through the angled capillary in such a way that it cannot be reflected back into the fiber. This light is lost to the system.
The light emitted in the reverse direction is directed back towards the WDM. This light is at a different wavelength from the pump light introduced by the laser diode. The WDM typically is optimized to separate the two wavelengths. Light from the EDF is at a wavelength such that it gets coupled into the fiber leg that is not connected to the laser diode. This light, which is broadband in nature, is then coupled into the fiber optic gyro.
To change this configuration to a double-pass system, the angled capillary is replaced with a reflector that preferably is a dichroic mirror. The reflector may alternatively be a Bragg grating or a straight cleave on the end of the optical fiber. The dichroic mirror causes the light in the forward direction to be reflected back in the reverse direction. This has the advantages that a shorter length of EDF can be used without losing efficiency, more power can be realized from the broadband fiber source for a given pump power, and the wavelength of the broadband source is more stable over temperature.
The problems as stated above with a double-pass configuration arise when the source is used as the optical signal source for a fiber optic rotation sensor. Half the light from the gyro is directed back into the broadband source because the gyro uses a fiber optic multiplexer (MUX) to separate the input light from the output light. This MUX is typically a 50/50 optical coupler. The light that is directed back into the broadband source acts to reduce the efficiency of the erbium fiber by a process called gain depletion. This problem is encountered with both the single reverse pump and double-pass configurations.
The second problem encountered is especially prevalent in the double-pass configuration. When all three fiber optic gyros are tied together for use as a three-axis inertial measurement unit, IMU, all three axes use the same broadband light source. This allows for a significant cost savings since three individual light sources are not needed. In a double-pass configuration with a mirror at the end of the EDF the light from one gyro that is reflected back due to the feedback described above can be reflected back into any one of the three axes. This creates a cross-coupling error term for each of the three gyros.
A third problem arises especially when trying to use the double-pass broadband source in a high power mode. In this case back reflections in the erbium gain fiber create etalons that collapse the broadband spectrum into a very narrow spectrum. This condition is unstable and introduces bias and scale factor instabilities into the gyro. Furthermore when the spectrum collapses the coherence length of the source becomes very long that allows for the introduction of polarization non-reciprocal (PNR) bias errors.
There is a need in the art for a double-pass optically pumped optical signal source that does not have backward reflections of the pump light into the gain fiber. To correct these problems back-reflections into the erbium-doped fiber must be eliminated.